1. Field of the Invention
The present invention relates to a pump-equipped liquid supply system for discharging fixed amounts of a liquid to a liquid tank in such a manner that the liquid held in the liquid tank is maintained constant.
2. Discussion of Background
Examples of conventionally known liquid supply systems for supplying a liquid to a tank containing the liquid to maintain the amount of the liquid in the tank constant are: (1) a liquid developer supply unit which is incorporated into an electrophotographic copying machine and from which a liquid therein is discharged by its own weight from the unit into the electrophotographic copying machine; and (2) a liquid supply unit which is pre-pressurized to discharge a liquid into a tank.
However, there are many problems associated with such conventional liquid supply systems. For example, it is difficult to control the supply of fixed amounts of a liquid by using the aforementioned unit (1), and a special device is required for the application of a constant pressure for the pre-pressurizing in the unit (2).
The applicant of the present application has proposed a pump-equipped liquid developer supply system as shown in FIG. 5, capable of reliably supplying a fixed amount of a liquid developer held in a pump-equipped liquid developer supply vessel to a developer tank at each operation of the pump without providing any pressurizing means, as Japanese Utility Model Application 3-26080 (which corresponds to a copending U.S. application No. 07/800,490) now U.S. Pat. No. 5,255,828, Oct. 26, 1993.
In FIG. 5, reference numeral 1 indicates a liquid developer vessel, reference numeral 20, an air displacement pipe; reference numeral 21, a cylinder cover through which the air displacement pipe 20 passes; reference numeral 22, a cylinder; reference numeral 23, a valve for a liquid developer introducing port; reference numeral 24, a piston cover; reference numeral 25, a piston; reference numeral 26, a lower space portion of the cylinder 22; reference numeral 27, a first liquid developer suction hole through which the liquid developer is sucked from a liquid developer supply vessel; reference numeral 27', a second liquid developer suction hole; reference numeral 40, a piston shaft; reference numeral 28, a liquid developer discharge conduit which is formed within the piston shaft 40 for discharging the liquid developer to a developer tank 50; reference numerals 29 and 30, sealing members; reference numeral 32, a liquid developer suction valve; reference numeral 33, a liquid developer inlet valve; reference numeral 34, a liquid developer inlet valve; reference numeral 35, a spring which urges the piston 25 downward; reference numeral 36, an upper space formed above the piston cover 24; reference numeral 41, an air displacement port; and reference numeral 51, a solenoid.
The operation of this pump-equipped liquid developer supply vessel is as follows: when the piston shaft 40 is pushed upward in the direction B shown by the arrow by the action of the solenoid 51, as indicated in FIG. 5, the liquid developer suction valve 32 opens so that a fixed volume of the liquid developer in the developer supply vessel 1 is drawn into the lower space 26 in the cylinder 22. At the same time, the liquid developer inlet valve 34 is opened and the liquid developer is drawn out from the upper space 36 in the cylinder 22, passes through the liquid developer conduit 28 in the piston shaft 40, and is discharged therefrom to the liquid developer tank 50.
Then, after the piston shaft 40 has completed its upward movement, when the piston 25 is pressed down in the direction A by the action of the spring 35, as indicated the valve 23 is opened and a fixed amount of the liquid developer is drawn into the upper space 36 in the cylinder 22. At the same time, the liquid developer inlet valve 33 is opened and the liquid developer is discharged into the developer tank 50 through the liquid developer discharge conduit 28 formed within the piston shaft 40.
As the liquid developer is discharged from the supply vessel 1 to the developer tank 50, the air in the tank is introduced into the bottom of the liquid developer supply vessel through the air displacement port 41 and through the air displacement pipe 20.
In the above-described pump-equipped liquid developer supply system shown in FIG. 5, the liquid developer is simultaneously drawn into the pump portion of the supply vessel 1 and discharged to the developer tank 50 by the action of the piston in either of the directions A or B.
In the above pump-equipped liquid developer supply system, however, when the internal pressure of the liquid developer supply vessel is extraordinarily increased, there is a problem in that the liquid developer leaks from an outlet of a liquid outlet 28 and/or from an air displacement port 41.
More specifically, with reference to FIG. 5, when the internal pressure of a pump-equipped liquid developer supply vessel 1 filled with a liquid developer is increased, for instance, by the increase of the temperature therein, the liquid developer enters the cylinder 22 through the first liquid developer suction hole 27 and the second liquid suction hole 27' and leaks into the developer tank 50 through the liquid developer discharge conduit 28.
In addition, when this kind of pump-equipped liquid developer supply vessel 1 is incorporated into an electrophotographic copying machine, there is a problem in that the liquid developer in the developer supply vessel 1 enters an air displacement pipe 20 and leaks from the air displacement port 41.